This invention generally relates to acoustic stacks for ultrasonic transducers and methods for manufacturing those stacks.
Ultrasonic transducers incorporate piezoelectric ceramics which are electrically connected to a pulsing-receiving unit in the form of an ultrasonic test unit. Portions of the surfaces of the piezoelectric ceramics are metal coated (with a sputter plating process), forming electrodes (signal and ground), which are connected to the ultrasonic test unit. During operation, an electrical waveform pulse is applied to the electrodes of the piezoelectric ceramic, causing a mechanical change in ceramic dimension and generating an acoustic wave, which is transmitted through a material such as a metal to which the ultrasonic transducer is coupled. Conversely, when an acoustic wave reflected from the material under inspection contacts the surface of the piezoelectric ceramic, it generates a voltage difference across the electrodes that is detected as a receive signal by the ultrasonic test unit or other signal processing electronics.
The amplitude, timing and transmit sequence of the electrical waveform pulses applied by the pulsing unit are determined by various control means incorporated in the ultrasonic test unit. The pulse is generally in the frequency range of about 0.5 MHz to about 25 MHz, so it is referred to as an ultrasonic wave from which the equipment derives its name. By tracking the time difference between the transmission of the electrical pulse and the receipt of the electrical signal and measuring the amplitude of the received wave, various characteristics of the material can be determined. Thus, for example, ultrasonic testing can be used to determine material thickness or the presence and size of imperfections within a material.
Many ultrasonic transducers are phased arrays comprising single or multiple rows of electrically and acoustically independent or isolated transducer elements. A linear array of independent transducer elements can form what is referred to as a transducer pallet comprising a plurality of independent transducer elements. In these types of transducers, each transducer element may be a layered structure comprising a backing block, flexible printed circuit board (“flex circuit”), piezoelectric ceramic layer, and acoustic matching layer. This layered structure is often referred to as an acoustic stack. The various components of the acoustic stack can be bonded together using an adhesive material (e.g., epoxy) and high pressure in a lamination process.
Typically, one or more flex circuits are used to make electrical connections (signal and ground) from the piezoelectric ceramic to the ultrasonic test unit, or to a bundle of coaxial cables that ultimately connect to the ultrasonic test unit or other signal processing electronics. Prior to bonding with the flex circuit, the piezoelectric ceramic can be processed to produce a plurality of spaced columns/posts or planes projecting from a solid piece of the ceramic material which is unaffected by the processing. This unaffected solid piece of ceramic is referred to as the ceramic backbone. After the plurality of spaced columns or spaced planes, also referred to as a diced ceramic, has been formed, the spacing between the columns or planes is filled with an epoxy. Sufficient epoxy is applied to form a continuous layer of epoxy, or epoxy backbone, overlying the diced ceramic and opposite the ceramic backbone.
The ceramic backbone and epoxy backbone are removed by grinding below the backbones into the diced ceramic, removing a small portion of each post or plane, resulting in a plurality of ceramic posts embedded in epoxy. Both sides of the ceramic workpiece are then finish ground, resulting in the ceramic posts being depressed typically from about 15,000 to about 30,000 Angstroms below the epoxy. The ceramic workpiece is then cleaned and sputter plated, providing a very thin plating over the surface having a thickness is about 15,000 Angstroms. Because the ceramic posts are depressed below the surface of the epoxy, it is possible that the sputtering process may not provide a uniform coating of the surface, particularly along the perpendicular surfaces extending between the parallel planes of epoxy and ceramic material. In addition, since the sputter plating operation is performed at temperatures of about 120° C. (about 250° F.), the epoxy is free to expand unrestrained above the ceramic posts or planes. Even though this expansion is small, because of the thinness of the plating deposited by the sputter plating process, it can be sufficient to damage the thin plating extending in the vertical direction along the epoxy between the ceramic posts and the horizontal surface of the epoxy, causing poor performance of the ceramic, such as low capacitance. It is therefore desirable to provide a piezoelectric ceramic layer wherein the ceramic posts are above rather than depressed below the epoxy.
One method of forming a linear array of independent transducer elements is to laminate a single acoustic stack of a certain length that is then diced into separate transducer elements, each element laminated together to form an independent acoustic stack. A dicing saw is used to form parallel element isolation cuts or kerfs, with each cut passing completely through the acoustic matching layer, the piezoelectric ceramic layer, the flex circuit, and extending only partially into the backing block. These kerfs provide electrical and acoustic isolation between the independent transducer elements. The need for dicing the acoustic stack complicates and lengthens the manufacturing process. It is therefore desirable to provide electrical and acoustic isolation between transducer elements without the requirement of dicing the acoustic stack.